The present invention relates to pressure sensitive paints, and more particularly to a method and apparatus of correcting for the temperature sensitivity of pressure sensitive paints.
Many vehicles, such as automobiles and aircraft, have to operate in an atmosphere of dense gas. Therefore, it is highly desirable to optimize the aerodynamics of these vehicles. Furthermore, with certain vehicles such as aircraft, certain aerodynamic interactions must be known to assure that the aircraft will perform properly and be capable of being flown in a controlled manner. To this end, several methods have been used to measure pressure on the surface of models of different vehicles to assure that they will be able to operate efficiently and properly during travel. Wind tunnels are often used to simulate a vehicle traveling at a particular speed through the atmosphere. Once the model is placed in the wind tunnel, air is moved at a particular speed over the model to test how the model reacts to the wind speed. One particular measurement is the pressure produced over the various surfaces of the model during the tests in the wind tunnel. To measure these different pressure points, many techniques have been developed.
Generally, wind tunnels use several mechanical devices to measure the pressure changes along the surfaces of the models placed in the wind tunnel. Each of these numerous mechanical devices are affixed to different electrical leads, which electrical leads are coupled to a computer to produce a representation of the pressure changes produced on the model by the wind in the wind tunnel. These systems are often cumbersome, time consuming, and hard to set up and take down for each model being tested.
Other methods of detecting pressure changes include pressure sensitive paints (PSP). These PSPs are able to luminesce at a particular wavelength when pressure is applied thereto. Generally, PSPs luminesce when a particular type of light energy is applied to them at a particular wavelength. Also, the PSPs are oxygen permeable. In the wind tunnel, the PSP, being oxygen permeable, receives a particular amount of oxygen depending upon the pressure being applied to an area. When a particular wavelength of light is applied to the PSP, it luminesces. If oxygen is in the matrix of the PSP, the oxygen absorbs a certain amount of the luminescent energy that would otherwise be emitted, thereby changing the luminescence of the PSP depending upon the amount of oxygen that is absorbed into the PSP layer. Therefore, a pressure of oxygen is determined and interpreted therefrom. A general pressure of the air around the particular area of the model being measured is determined from the known concentration of oxygen. Often the PSPs are sensitive not only to pressure, but also temperature. Therefore, temperature can effect the degree of luminescence from the PSP. Due to this temperature sensitivity, pressure cannot be accurately determined from the luminescence of the PSP alone.
Several methods have been attempted to correct for the temperature sensitivity of PSPs with only marginal success. One method is to simply shorten the amount of time between when measurements are taken. That is, the time between a zero or initial reference, that being when no wind or pressure is being applied to the model, and when the maximum amount of wind pressure is being applied to the model. However, this does not actually correct for temperature, but rather simply reduces the amount of temperature change that occurs and reduces the error of the PSP to one that is acceptable for the tests.
Other methods have attempted to mix several different sensors into a single film wherein each is affected by temperature, pressure, or other factors. The mixing of all of the phosphorescence species into a singular film, however, has reduced the temperature error of the PSP only marginally. Furthermore, it is difficult to provide each of the particular phosphorescent species in a singular film since it increases the effort and cost necessary to produce such a film.
Additionally, attempting to correct for the temperature change through data conversion after the test data has been taken during a test has been tried. Again, this method is long and arduous and only corrects for a certain amount of the temperature related error. Additionally, this method is only marginally helpful in correcting for the temperature change in the PSPs.
The present invention relates to a system that can adjust or take account of the temperature sensitivity of a pressure sensitive paint to produce a measurement that is nearly error free due to temperature sensitivity of the pressure sensitive paint.
A first preferred embodiment of the present invention includes a system for detecting at least two physical characteristics near a surface being tested. A first luminescent film, capable of emitting light having a wavelength in a first discrete range, is placed on the surface. A second luminescent film, capable of emitting light having a second wavelength in a second discrete range, is placed over the first film. A radiation source which emits radiation able to excite the first luminescent film and the second luminescent film is focused on the films. An analysis system detects the brightness of the light emitted by the first luminescent film and the brightness of the light emitted by the second luminescent film. The first luminescent film and the second luminescent film are placed on the surface substantially coplanar and the first luminescent film is substantially transparent to the light emitted from the second luminescent film.
A second preferred embodiment of the present invention comprises a method of more accurately determining a pressure on a surface being tested. The method comprises measuring the brightness of light emitted by a first film sensitive to temperature, which covers the surface being tested. Additionally, the brightness of light emitted by a second film, which is sensitive to both pressure change and temperature, and which covers the first film, is measured. At least a first measurement of the brightness of light emitted by each of the films is taken. Then the surface is made to experience a pressure and temperature change. At least a second measurement of the brightness of light emitted by each of the films is taken. Finally, an accurate determination of the pressure experienced by the surface is obtained by comparing the first measurements and the second measurements.
A third preferred embodiment of the present invention includes a system to determine the pressure over an area of a surface. A member under test has a first film applied so as to surround the member, wherein the first film comprises a sensor that is adapted to emit light at a first wavelength. A second film is placed over the first film to surround the first film. The second film comprises a second sensor adapted to emit light at a second wavelength. The first film is transparent to the second wavelength. A test is performed and each sensor in each film emits light at a particular brightness which is measured.